The effect of calcium chloride (CaCl 2 ) on tricalcium silicate (C 3 S) hydration was investigated by scanning transmission X-ray microscopy (STXM) with Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra and 29 Si MAS NMR. STXM is demonstrated to be a powerful tool for studying the chemical composition of a cement-based hydration system. The Ca L 3,2 -edge NEXAFS spectra obtained by examining C 3 S hydration in the presence of CaCl 2 showed that this accelerator does not change the coordination of calcium in the calcium silicate hydrate (C-S-H), which is the primary hydration product. O K-edge NEXAFS is also very useful in distinguishing the chemical components in hydrated C 3 S. Based on the Ca L 3,2 -edge spectra and chemical component mapping, we concluded that CaCl 2 prefers to coexist with unhydrated C 3 S instead of C-S-H. In Si K-edge NEXAFS analysis, CaCl 2 increases the degree of silicate polymerization of C-S-H in agreement with the 29 Si CP/MAS NMR results, which show that the presence of CaCl 2 in hydrated C 3 S considerably accelerates the formation of middle groups ( Q2 ) and branch sites ( Q3 ) in the silicate chains of C-S-H gel at 1-day hydration.

School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin 150090, China, Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA 94720, USA

School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin 150090, China

Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA 94720, USA

Department of Civil and Environmental Engineering, University of California at Berkeley, Berkeley, CA 94720, USA, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamasaki, Noda, Chiba 278-8510, Japan